This invention relates to predistortion circuits for radio frequency power amplifiers and particularly to such circuits incorporating dual gate FET's.
In communications systems involving power amplifiers there is typically distortion of both phase and gain in passing through the amplifier due to amplifier non-linearities. Such non-linearities cause intermodulation distortion which, in communications systems, is highly undesirable. Such power amplifiers producing typically a few watts of output power are usually traveling wave tube amplifiers (TWTA) or the newer solid state power amplifiers (SSPA). Of the two types of power amplifiers, the TWTA suffers from greater distortion. The inherent linearity of the SSPA, especially the GaAS field-effect transistor (FET) amplifier, is much better. Therefore the techniques for reducing distortion in SSPA's need not be as complex as the technique applicable to TWTA's.
An intermediate frequency predistortion technique for SSPA's has been reported in the article entitled "Complex Predistortion of High Frequency Signals", Electronics Letters, Vol. 9, p. 565, 1973.
The present invention relates to a radio frequency predistortion technique involving a dual gate FET. It is known to provide to the control gate of a dual gate FET a signal which is a function of an input signal applied to the signal gate of the FET to alter the gain of the FET as a function of input power so the FET produces constant output power in the milliwatt range regardless of the input power. Such a circuit is reported in an article entitled "Dual-Gate MESFET Variable-Gain, Constant-Output Power Amplifier" by M. Kumar and H. Huang, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-29, No. 3, March 1981, pp. 185-189.
More recently it has become known that the gain and phase of a dual gate FET can be controlled using a matching network having an inductive impedance at one of the gates of the dual-gate FET and applying a variable d.c. bias to the second gate of the dual-gate FET. This investigation is reported in an article entitled "Dual Gate GaAs M.E.S.F.E.T. Phase Shifter with Gain at 12 GHz," by C. Tsironis and P. Harrop, Electronics Letters, 3rd July 1980, Vol. 16, No. 14, pp. 553-554. In this investigation, however, the signal applied to the control gate of the FET is not related to the value of the input signal applied to the signal gate. The investigation did not involve connecting the output of the dual gate FET to a following circuit nor was it concerned with adjusting the gain of the dual gate FET using the control gate.